Vesicular structures occur frequently in living cells. Among their more important functions are the storage, transport, and release of neurotransmitters. Successful functioning depends on the inherent stability of vesicular membranes and on the tendency of these membranes to undergo vesicle-cell fusion. Our objectives are the attainment of an understanding of molecular properties that lead to vesicle stability and membrane fusion. We plan to study these properties on model phospholipid vesicles of varying composition. These models mimic natural systems both in vesicle size distribution and vesicle fusion properties. Size distribution which vary as a function of composition, will be determined by light scattering, sedimentation velocity and analytical gel chromatography. Experimental observations can be compared to predictions based on theoretical models for molecular interactions. Interactions at a molecular level will also be characterized by carbon-13 magnetic resonance spectroscopy, especially as reflected in spin-lattice relaxation times. Fusion of vesicles can be induced with a small percentage of free fatty acid in homogeneous vesicle systems at an appropriate environmental temperature, or with the addition of a polyene antibiotic to cholesterol - containing vesicles. Preliminary experiments suggest the separation of a catalytic site in each case. This suggestion will be verified by a variety of magnetic resonance experiments including those which involve induced changes in lipid spin-lattice relaxation times by electron spin-labeled acids, and those which involve changes in HI-F19 nuclear Overhauser effects. The results of these experiments will be used to evaluate possible stability or fusion altering agents found in natural systems.